Lion Redox Flow Battery

Redox flow batteries (RFB) are new electrical energy storage systems (the flow redox concept was developed at NASA in the 70’s) based on redox regenerative reactions converting chemical into electrical energy. Their architecture consists of a cell stack and separate rechargeable electrolytes reservoirs. The power and energy rating are decoupled: the energy capacity is a function of the electrolyte volume (and active ions concentration) while the power is a function of the surface area of the electrodes. During operation the electrolytes are pumped through the cell stack (electro chemical reactor) in which electricity is produced by a redox reaction (process in which atoms have their oxidation number changed). The two electrolytes are separated by ion permselective membranes which permit the flow of ions but prevent the mixing of the liquids. As the ions flow across the membrane an electrical current is induced in the inert electrodes. Because the RFB’s employ heterogeneous electron transfer rather than solid-state diffusion they should be more appropriately called cells than batteries. Actually, the European Patent Organization classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).

RFB’s have features which make them best energy storage solution for utility scale and renewables applications:

– They are truly closed systems with regenerative electrolytes (which do not have to be replaced).
– The inert electrodes confer exceptional cycle life (over 10,000 cycles estimated life in case of Lion).
– The recharge / discharge time ratio is close to 1/1 while in case of conventional batteries is 5 /1.
– Once charged, the flow battery’s electrolyte remains fully charged with low self-discharge.
– They are safer and more scalable than lithium-ion batteries, and can withstand extreme temperatures and periods of idleness, which makes them well suited for renewable energy applications.
– In addition to electrical recharging, flow batteries can be rapidly replenished by simply replacing the electrolyte.
– They can increase the storage capacity by increasing the size of the electrolyte tanks which makes them suited for utility scale applications. Installation costs per unit of energy decline as the system tanks grow larger
– They can cycle rapidly and deeply, without significant self discharge, have fast reaction kinetics with 1/1000s charge to discharge modes and no need for “equalization”. Due to the rapid response and the low operating costs when idle, they are well suited for voltage and frequency regulation, load leveling, spinning reserve.
– They have full to 25 % discharge while the lead acid batteries’ depth is from full to 80% state of charge.
– They have high round trip efficiencies (65 – 75%) as compared with the lead acid batteries (45%).
– They have the lowest ecological impact of all conventional energy storage technologies including hydro.
– Disadvantages of existing RFB’s are lower specific energy (making them heavy for mobile applications), low specific power (making them expensive for stationary applications) and the need for sophisticated electronics.

Other developed flow batteries include hybrid metal hydride FB. The hybrid FB use one or more electroactive components deposited as a solid layer. Proton flow batteries integrate a metal hydride storage electrode into a reversible proton exchange membrane fuel cell. Metals less expensive than lithium can be used and provide greater energy density.

Membraneless batteries employ a laminar flow in which two liquids are pumped through a channel.

R & D on Organic Redox Flow (ORF) batteries has emerged in last few years promising to overcome drawbacks preventing the deployment of existing inorganic batteries (toxic and expensive electrolyte materials). The ORF’s batteries are using sustainable organic redox active molecules, free of resources limitation and enabling unlimited combinations of anode – cathode materials (aqueous and non aqueous). ORF’s still have challenges to resolve related to energy density, cost, radical induced side reactions, electrolyte crossover, and limited life time.

Lithium-ion batteries (Li-ion or LIB) are currently popular rechargeable batteries in which lithium ions move from the negative to the positive electrode during discharging / charging. They are not only used for portable electronics (high energy density, tiny memory effect and low self-discharge) but they are becoming a common replacement in larger format applications for the traditional lead–acid batteries. The lightweight lithium-ion battery packs tend to be preferred to the heavy lead plates and acid electrolyte traditional batteries. Chemistry, performance, longevity, safety and cost vary.(lithium–sulfur LIB’s promises the highest performance / weight ratio). LIB’s can pose safety hazards since they contain flammable electrolytes and may be pressurized. Specific costs of the LIB is over $450 / Kwh. The well advertised Tesla Lithium Ion Powerwall battery (nickel – manganese – cobalt chemistry) costs over $400 / Kwh without installation hardware.

Examples of developed RFB’s are vanadium, polysulfide, bromide and uranium based. RFB’s have not proliferated yet due to low energy density (70 Wh/l compared with over 200 Wh/l for lithium iron phosphate). New research reports redox couples with higher energy density (zinc-polyiodide hydrogen-bromine and hydrogen- bromate). Even if the multi-billion dollar potential of redox flow batteries storage business is currently limited by cost and reliability, specific costs of redox flow batteries are expected to get substantially lower then those of lithium ion batteries and flow batteries are expected to become the backbone of future electricity systems.

Advantages of the Lion Redox Flow Battery

Lion has developed a battery in the flow – redox category with cost and performance advantages. The battery is using proprietary components: redox couple electrolytes, low cost and durable permselective ion exchanging membrane with proprietary production process, composite electrode structure and electrode irrigation architecture, hollow polymer tubing and special adhesives used in the stack structure.

The redox couple used is chemically stable, with high solubility of both oxidized and reduced species, and fast redox kinetics. The electrolytes do not generate colloids, gels or suspensions which could alter the operation of the battery and require frequent membrane replacement. There is no electrolyte cross mixing across membrane.

The electrolyte contains substances which are low cost and with no resource limitation allowing the proliferation of the Lion batteries without the risk of raw materials shortages. For other batteries metal could actually comprise a majority of the battery cost (in case of the vanadium flow battery the estimated cost of vanadium represents approx. $50/kWh – $100/kWh of the total battery cost)

The electrolyte requires simple production technologies as opposed to the case of some existing flow batteries with complex production (Chromium, Vanadium, Polysulfates etc.) which could require complexing agents further increasing costs. The production of the Lion electrolytes does not require high energy consumption which cause high emissions (as for example in the case of vanadium with 3,711 GJ / t energy consumption).

The electrolyte is benign, non toxic, non flammable and fully recyclable. The system operation does not release toxic pollutants while flow batteries like Zinc Bromide or Vanadium present a high toxicity risk using highly-corrosive sulfuric acid (or combined with hydrochloric acid).

The Lion permselective membrane has a favorable combination low electrical resistance – high mechanical resistance. It can be produced using a proprietary process at a fraction of the cost of Nafion (Dupont) membranes currently used by flow batteries. A high resistance low cost membrane is important since membranes are often the costliest and least reliable components of batteries, as they can corrode with repeated exposure to reactants.

The battery offers great scalability for utility applications due to the stable electrolyte allowing larger storage tanks and to the mechanical resistance of the membrane allowing larger membrane dimensions.

Due to its indefinite electrolyte service life and the large range of service temperatures (-25°C to +80°C) the Lion battery is suitable for remote locations with extreme temperatures.

Lion batteries contain modular components which can be produced in a continuous flux and they can cover a large range of power capacities (5kW – 20MW and larger), and therefore a large range of applications (renewables, distributed power, energy management, premium power etc).

The average specific acquisition cost for conventional batteries is over $250/Kwh, for lithium-ion batteries over $400/Kwh and for the existing flow batteries over 350 /Kwh. The industry and government labs consensus is that for flow batteries to be widely adopted, their price must be under $100 / Kwh. The Lion flow battery estimated specific production cost will be under $80 / Kwh capacity. But the long cycle life of the Lion electrolyte and membrane would actually translate into an even lower actual specific cost.

Most of Lion’s research has been focused so far on the redox couple. Work will continue on reducing pumping and shunt current losses and increasing power density by a better electrode irrigation. For electronics and automation Lion is considering a possible cooperation with university research labs.

The costs of electricity storage technologies will ultimately play the critical role in their deployment. The specific cost for the Lion battery will be the lowest in the industry due to the low cost redox couple substances, the long cycle life of the electrolyte and membranes and other proprietary architecture features.

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